1. Field of the Invention
The present invention relates to a camshaft with cams, more particularly, it relates to a process for producing a camshaft with cams having a surface hardened layer formed by a remelting chilling treatment of high density energy, such as a tungsten inert gas (TIG) arc, a laser beam, plasma arc, and an electron beam, on a sliding cam surface of each cam.
2. Description of the Related Art
In the production of a camshaft, recently, irradiation of high density energy, such as a TIG arc, a laser beam, a plasma arc, and an electron beam, to a sliding cam surface of cams of a camshaft is utilized so as to form a surface hardened layer having a superior wear resistance (cf., e.g., GB-A-2004613, Japanese Examined Patent Publication (Kokoku) No. 57-6494 based on priority of DE Patent Application No. 2741567.4). The present coinventors (H. Nonoyama, T. Fukuizumi, and A. Morita) have proposed "a method of producing a camshaft", (in U.S. Ser. No. 730,484, filed on May 6, 1985, corresponding to EP-A-0161624) using high density energy irradiation. The surface hardened layer of each cam is formed by the steps of applying high density energy, e.g., a TIG arc, on a sliding cam surface to melt the irradiated portion and, immediately after irradiation, cooling rapidly the heated portion including the molten portion by dissipating localized heat through the cam and the camshaft body (i.e., allowing the heated portion to self cool), so that the surface hardened layer consists of a chilled layer and a hardened layer (i.e., a martensitic layer). In order to form the surface hardened layer on each of the cams of a camshaft, for example, an electric arc is generated between a tungsten torch and a sliding cam surface, simultaneously, the camshaft is rotated on its longitudinal axis and is reciprocated in a direction parallel to the axis of the camshaft at a distance of less than the width of the cam, and the gap between the cam and the tungsten arc torch is maintained at a constant distance. Accordingly, the arc oscillates over the cam surface and the continuously remelted zigzag bead is joined so as to form a single bead layer. It is possible to reciprocate the tungsten torch instead of the camshaft.
If the position of the cam to the tungsten arc torch is off from the prescribed position in a direction parallel to the axis of the camshaft, the obtained surface hardened layer shifts from the proper location. As a result, defects in the camshaft occur, namely, a melt-down end portion is formed at the edge of the cam, and an unhardened portion may appear on the sliding cam surface.
For production of such remelted chilled camshafts, generally, the following two processes are adopted.
First, all the cams of a camshaft are subjected to a remelting chilling treatment in a single station. In this case, plural torches for high density energy irradiation are used to treat some cams and then the torches or the camshaft are shifted to a next prescribed position for treating some other cams. Since the remelting chilling treatments are repeated for all the cams, the total time of the treatment is relatively long. When a large number of camshafts are produced, such a relative long time of the remelting chilling treatment may limit the productivity of the production line. Furthermore, if the production halts during the remelting chilling treatment, the longitudinal dimension (i.e., the overall length) of the camshaft to be treated varies due to the thermal shrinkage by cooling. When the treatment restarts, there is liable to be a melt-down end portion.
Second, plural stations, each of which has plural torches for some cams, are provided so as to form a transfer production line. The torches in the stations are arranged for the cams of a camshaft, respectively. In this case, plural camshafts are set in the plural stations, respectively, and the remelting chilling treatments in the stations are almost simultaneously carried out. Thus, the number of the produced camshafts per unit period is increased, so that the second production process is suitable for mass production. However, a cool camshafts is, generally, preheated at a temperature of approximately 400.degree. C., and camshafts heated by the high density energy (e.g., TIG arc) are controlled to have a temperature of approximately 400.degree. C., before the remelting chilling treatment. Since the high preheating temperature of 400.degree. C. entails a large degree of thermal expansion in a longitudinal direction of the camshaft, a position of the torch for a cam must be controlled in consideration of the thermal expansion quantity. If the production halts, which brings about the cooling of the camshafts, the cool camshafts are subjected to a remelting chilling treatment without preheating so that melt-down end portions are liable to occur. In order to prevent the formation of melt-down end portions, a heating device or heating zones for heating or keeping the camshafts at 400.degree. C. and a controlling system for the heating device or the heating zones are provided in the transfer production line. If necessary, a device for transporting the camshafts between the heating device and the stations for remelting chilling is also provided. Such additional devices and system complicate the production apparatus and increase the initial cost. Furthermore, concerning the quantity of the surface hardened layer of the camshaft, the chill structure of the chilled layer of the camshaft preheated at approximately 400.degree. C. is similar to the coarse cementite structure of a chilled layer of a camshaft produced by casting molten metal into a mold provided with chillers (i.e., chilling blocks) for cams. Although a remelting chilling treatment using high density energy can form a superior fine cementite structure of a chilled layer, the preheating at approximately 400.degree. C. prevents the formation of it. When a camshaft preheated at a temperature of more than 250.degree. C. is subjected to the remelting chilling treatment, bainite occurs in a martenstic hardened layer under the chilled layer. The coarse cementite and bainite are insufficient to secure superior wear resistance and scuffing resistance of the surface hardened layer of cams.